1. Field of the Invention
The present invention generally relates to ATM (Asynchronous Transfer Mode) networks, and more particularly to an apparatus and method for controlling cells which are input to an ATM network.
2. Description of the Prior Art
Recently, there has been considerable activity in the development of high-speed communication networks capable of handling a large amount of data. One of such high-speed communication networks is an ATM network.
A subscriber (terminal) requests a band (frequency range) used by the subscriber to the ATM network when setting up a call. In response to this request, the ATM network calculates a necessary band based on the requested band, and determines, on the calculated band, whether or not the call from the subscriber should be permitted. If the number of cells which are input to the ATM network greatly deviates from the number of cells corresponding to the requested band due to an evil intention or an unexpected matter, some of the input cells will be frequently destroyed in the ATM network. This will affect other calls transferred via the ATM network.
In order to eliminate the above problem, a supervisory device is provided at a subscriber interface unit of the ATM network. Referring to FIG. 1, an ATM exchange 10 includes a subscriber interface unit 13 and an ATM switch 14. The subscriber interface unit 13 is composed of a subscriber line terminating unit 11 and a cell input control unit 12. The cell input control unit 12, which corresponds to the above-mentioned supervisory device, discards input cells if the number of input cells greatly deviates from the requested number of cells. The cell input control unit 12 monitors the number of input cells which are supplied to the ATM switch 14. When the number of input cells is smaller than the requested number of cells, the cell input control unit 12 transfers the input cell from the subscriber line terminating unit 11 to the ATM switch 14.
The cell input control unit 12 operates in the following manner. The cell input control unit 12 includes a counter, which increases the counter value by "1" each time one input cell is received. The counter value in the counter decreases by "1" each time a predetermined time elapses. When the counter value exceeds a predetermined threshold value, the input cell is discarded. The above operation can be written as follows: EQU FA(t)=G(t)-INT(t/T) (1)
where FA(t) denotes the counter value, INT(X) is a function of omitting the numbers on the right side of the decimal point, G(t) denotes the number of input cells obtained during time t, and T is a period at which the counter value FA(t) is decreased by "1". When FA(t) &lt;.alpha. (.alpha. is a tolerable value), the input cell is permitted to be input to the ATM switch 14. When FA(t) &gt;.alpha., the input cell is discarded.
In general, the ATM network has a characteristic such that a plurality of calls exist in one interface. However, the conventional cell input control unit 12 is not designed to process a plurality of calls in one channel.